Is there a UNIX/Linux computer? It's both a yes and no answer, but if you want just one, then it's No.
UNIX and Linux variants are operating systems, just like Mac OSX and Microsoft Windows.
Apple create both the underlying hardware and the operating system. Together these make a Mac.
Microsoft does not create it's own hardware for windows (semantics - yes, they make mice, and tablet PCs, etc, but leave this aside for now)
But with UNIX and Linux - well, these are quite broad families of operating systems - check out http://en.wikipedia.org/wiki/List_of_Linux_distributions and the picture at the right for an example, and http://en.wikipedia.org/wiki/List_of_Unix_systems
Windows is quite hardware agnostic - it doesn't care if you run it on an AMD or Intel or Motorola CPU, an Asus or Gigabyte motherboard, etc, etc.). As long as your computer's architecture (such as x86, x86-64, IA64, etc) matches the Windows version you are using, then it will work. Note that each "Edition" of Windows (such as Windows 7 Pro) can come in a few architectures, which is why you'll find different installation media for each version (have a look at the
Mac OSX is a little different. Mac does care what hardware you use it on. In fact, you can only use their own hardware. The operating system essentially does some checks on the hardware and won't boot or install if it's not Mac hardware. There are 'hackintosh' distributions of OSX out there which modify the behaviour of the operating system to stop caring so much.
Now, UNIX is not exactly a single operating system - there's heaps of variants but they are all similar in that they attempt to follow certain specifications (defined by various groups) on how the operating system should behave. Check out http://en.wikipedia.org/wiki/Single_UNIX_Specification and http://en.wikipedia.org/wiki/POSIX for some bed-time reading (literally).
Linux has multiple variants as well. They essentially mimic UNIX-like operating systems, but by using different underlying code. Linux and UNIX are similar in so many ways - but with underlying differences.
Now, are there "UNIX/Linux computers"?
Yes, there are. For example, most supercomputers use a proprietary variant of UNIX or Linux. For example, Cray's latest computers use Cray Linux Environment. IBM's Roadrunner used a variant of Red Hat Enterprise Linux (RHEL). You simply couldn't get Windows or Mac to run on one of those machines.
Coming back to reality, since Supercomputers are not exactly what you were asking about, you can get workstations which run UNIX. Sun microsystems did a few, Silicon Graphics used to do some (and I think they might still do), but HP definitely still make a few. Have a look at http://h10010.www1.hp.com/wwpc/ca/en/sm/WF04a/12132708-12132710-12132832-12132832-12132832.html for a couple of HP-UX workstation offerings.
So, to look at the example above, the HP c8000 workstation uses a PA-RISC processor architecture. This workstation will only run HP-UX (officially). It could probably run some other Linux/UNIX/BSD variants with a little work, but HP won't install it for you or support the workstation if you do.
So, have we found a UNIX computer here? Not really - we've found a HP-UX computer. It won't use other UNIX variants like AIX or SCO (though, again, you could probably get them to work with a little tweaking).